Conventional laser displacement measurement systems compare the phase shift of a modulated reflected laser beam, with respect to the modulated transmitted laser beam, to determine the displacement or range of a target.
These systems incorporate a radiation source which transmits radiation towards a target, where the radiation is reflected off the target and detected by a radiation detector. The transmitted signal is then compared with the received signal to determine the amount of phase shift between the two signals, which corresponds to the range of the target.
Since light travels at a finite speed, it takes a fixed amount of time for radiation to travel from the transmitter to the object being scanned and back to the detector. This finite amount of time induces a phase shift in the received beam which, as stated above, corresponds to the distance the target is from the transmitter. However, problems are encountered when the phase shift is greater than one period of the transmitted beam, since this makes it difficult to obtain the actual range of the target. Therefore, the modulation frequency of the radiation source is often chosen so that the wavelength of the modulation frequency is equivalent to the maximum range determinable by the measurement system. For example, if the wave propagating through free space at the speed of light has a wavelength of 100 meters, the maximum range of the measurement system would be 50 meters.
This leads to a system in which the lower the modulation frequency of the transmitted radiation, the longer the wavelength and, therefore, the longer the range. However, as the wavelength is increased, the system accuracy is decreased. Additionally, once the amount of phase shift is in excess of one period, ambiguity arises in determining the absolute range (i.e. distance to target) of the target being scanned. However, the system is still able to determine displacement (i.e. movement of the target) relative to the initial position of the target. This displacement measurement will function regardless of the fact that the system cannot determine the absolute range of the target.
In order to compensate for these shortcomings, systems have been designed which utilize multiple radiation sources, radiation detectors, and optical scan paths. This enables the system to use lower frequency, longer wavelength radiation to determine a coarse range and a higher frequency, shorter wavelength radiation beam to determine a fine range. However, these systems are overly complex as they require, essentially, two complete independent laser displacement measurement systems combined into one device, where the output of these two systems is then combined to generate an absolute range. Additionally, there is a substantial increase in the cost and the size of these systems, as they require redundant optics and electronics in order to function properly and the cost of high-speed electronics capable of processing high-frequency modulation signals is substantial.